The invention relates to watercraft and more particularly to the construction of such sport and recreational watercraft as paddle propelled watercraft, surf kayaks, surf skis, surfboards, sail boards and rowing sculls.
The principle forms of resistance which have an effect on the travel of a hull through water are skin friction, wave making and eddy making. These forms of resistance are not constant and vary with the speed of the watercraft, the hull shape and the smoothness of the surface or skin of the hull.
The skin friction of the hull is caused by the tendency of water to stick to the sides of a moving hull and to be dragged along with it. The skin friction can be reduced by smoothing the surface of the hull i.e. by polishing or constructing the hull from a smooth surfaced material, and by streamlining or keeping any projections on the hull to a minimum. The skin friction of the hull can also be reduced by keeping the wetted surface area of the hull to a minimum.
The skin friction of the hull surface is greater at the bow than it is aft of the bow. Thus, the skin friction of a hull section is reduced proportionately as the section is located further aft from the bow. Accordingly, a longer hull is faster than a shorter wider hull of the same volumetric displacement, i.e. a racing scull is faster than a dinghy having the same volumetric displacement.
As a hull moves forward in the water it preempts space and pushes water out and down away from the hull. As the hull vacates the space water rushes back to fill the void, thus creating waves. Two kinds of waves are formed by the moving hull; bow waves which move diagonally away from the hull and side waves which move alongside the hull and eventually recombine at the stern to cause the disturbance known as the eddy. These waves do nothing to propel the hull, and have the effect of increasing the hull's resistance. The shape of the hull is a major factor in such wave formation.
The most critical factor in reducing hull resistance is the design. There are principally two forms of hull designs, the displacement hull and the planing hull. A displacement hull always displaces the same amount of water. Whether the displacement hull is at rest or in motion its waterline does not change. Thus to obtain a faster displacement hull one must increase its waterline length relative to its volumetric displacement. By contrast a planing hull gradually rises out of the water after it has attained a critical speed, thus reducing the wetted surface area of the hull. As the planing hull rises it displaces less and less water, and its total hull resistance drops and its speed increases.
Other hull design concepts are also used to reduce hull resistance. For instance the hull design concept of a double-ender watercraft, having a bow and stern of similar shape, e.g. a kayak, decreases hull resistance by decreasing the turbulence of recombining side waves of the stern and thus decreasing eddy formation.
The increased interest in recreation and sports involving watercraft such as, surf and whitewater kayaking, and windsurfing has renewed interest in the design and hull construction of small watercraft. The importance of hull design is illustrated by the evolution of cruising type sailboards. Sailboards hulls have evolved from flat bottom planing boards and surfboard type boards with a longitudinal rise or rocker to semi-displacement hull forms having a displacement type hull with an extreme amount of bow rocker. Since the power used to propel such small craft such as sailboards is limited, hull design becomes a significant factor in enhancing hull efficiency and stability. It is apparent, however, that there are many trade-offs involved in the design of such watercraft. The same features that are necessary for instance to enhance speed often detract from the maneuverability and stability of the craft. In addition hull designs for efficient for use at high speeds e.g. a planing hull, may not be efficient for use at lower cruising speeds, or maneuverable at such speeds. Various designs have been proposed for the design of more efficient hull forms for such craft, e.g. Weiss, U.S. Pat. No. 4,434,737 and Cashmere, U.S. Pat. No. 4,538,540; however, these designs do not appear to be useful for the wide range of sail and paddle powered recreational and sport watercraft.
The present invention concerns a novel watercraft design and hull construction which provides stable maneuverable watercraft with low hull resistance. This invention is useful in the design and construction of a wide range of watercraft types including muscle-powered and sail-powered watercraft.